1. Field
This invention relates to electrical power generating systems and in particular alternating current electrical power generating systems. More specifically, this invention provides an electrical power generating system including regulating apparatus in which the output power frequency is fixed and is independent of the prime mover output shaft rotational rate,
2. State of the Art
As is known, it is highly desirable to maintain the frequency of the output of an alternating current electrical power generator stable and fixed. In the United States, 60 hertz (hz) is the typical design frequency for rotating electrical machinery as well as for the output of alternating current electrical generators supplying that machinery. It is desirable, if not necessary, to maintain the frequency of any electrical system including a 60 hz system as stable as possible because of impedance losses and other losses which arise in electrical motors and the like if and when the supply frequency varies.
In practice, generators today require a fixed rotational rate output from a prime mover. The prime movers may be either internal combustion engines, such as gasoline engines, diesel engines or gas turbines. Prime movers may also be steam driven turbines. Typically, the prime movers are operated at a fixed rotational rate so that the rotational rate of the output shaft driving the generator system is a given. The fixed output (e.g., 1800 rpm, 3600 rpm) is a design factor identified and related to the number of poles which in turn are preselected to generate a 60 hz output from the generator.
Given the variable load requirements of a generator, it may be stated that the prime mover (e.g., diesel engine) will operate on occasion, if not frequently, inefficiently as to power output and fuel consumption. That is, the engine will be required to operate at a fixed rotation rate to deliver output power which could be more efficiently delivered at a different and probably lower rpm for low system loads and at a higher rpm for high and peak system loads.
Operating a prime mover, be it a diesel engine, turbine or whatever. at a fixed output rotational rate can be deemed inconsistent with the mechanical operating characteristics of the prime mover. For example, fixed rotation rate operation does not maximize the efficiency associated with operation along or over the range of a power torque versus rpm curve for the prime mover. That is, the efficiency of a engine varies over its range of operating rotational rates. For example, a gasoline internal combustion engine may be regarded as having a mechanical efficiency which varies from a low rpm to a high rpm. In particular, the mechanical efficiency diminishes at the higher rpms. Thus, when operating the generator system using an internal combustion engine, it would be preferred for a low power output requirement to operate the engine at a low rpm to make maximum benefit of the higher mechanical efficiencies. See, for example, Mark's Mechanical Engineer's Handbook, Sixth Edition, MacGraw Hill, 1964, page 9-109, page 4-54, page 9-149, et. seq. Operating prime movers at a fixed rmp also results in additional wear and tear on the prime mover when such may not be necessary based on the power/load requirements of the prime mover/generating system. For example, in order to produce a 60 hz output, it may be necessary to operate a diesel engine or gasoline engine at 1800 rpm or 3600 rpm for power loads which may equate only to about 10% of the available power. Operating the engine at an rpm far above that necessary to produce the desired power simply and practically results in unnecessary wear and tear on the engine. In turn, breakdown and/or repair may be more frequent. The cost associated with breakdowns and with the tear and wear is an additional cost relating to the overall cost efficiency of operating a prime mover at a fixed shaft output rotational rate.
Operating prime movers at a designed or fixed rotational rate has in combination with variable electrical loads produced a series of complex problems associated with load variances. Speed regulation equipment associated with, for example a turbine, has a certain definitive response time associated with significant load variances. See, "Marks' Mechanical Engineers' Handbook", Sixth Edition, MacGraw Hill Co., 1964, pages 9-224 through 9-226. Upon the sudden increase or decrease of load associated with the generating system, the prime mover will either slow or increase in speed because of the load variance. The response time is the time within which the governor reacts to bring the turbine or other prime mover back to the design rotational rate. During this period, the frequency of the output power proportionally varies with the rotational rate while the prime mover reacts to come back to the design rotational rate. The variance in frequency has a distinct impact which can in some circumstances be quite costly. The impedance characteristic of the load (e.g., electrical motors) change with the frequency. In situations where increased load is suddenly applied to the generating system, the impedance may change such that motors which are part of the variable output load may suffer impedance related losses to the point that mechanical damage may be imparted to that particular motor. From the other point of view, sudden decreases in load resulting in increases in frequency may have adverse mechanical results with respect to increased speed. The load powered by the generating system may suffer mechanical damage and/or unwarranted wear and tear. Also, variances in frequency may have less than desirable results for electrical equipment which is designed to operate at a set frequency to perform such functions as timing related to frequency. In short, it is desirable, but has been previously unobtainable, to develop a generating system which operates at a fixed output frequency irrespective of a variable load requirement and in which the rotational rate or shaft rate of the prime mover is allowed to vary over a wide range to take advantage of cost/efficiency operating characteristics of the prime mover.
A variety of different efforts have been made to devise means resulting in frequency independence or frequency stability in generator operation. For example, U.S. Pat. No. 2,854,617 (Johnson) describes a brush-type frequency control device to provide a stable frequency generator output. The device operates on a heterodyne principle requiring reference frequency feedback from a tachometer which is connected to the prime mover shaft. The tachometer output is combined with a frequency generator output to generate beat frequency signals. The beat frequency signal is supplied through a phase shifting network and mixer to the generator or alternator to maintain the output frequency at a designed or preselected frequency. That is, the system so disclosed does not allow for a freely variable shaft rotational rate. Furthermore, it does not operate independent of the prime mover generator system, and has a finite and calculable response time during load varying conditions. To the extent that the mixer does not remove undesired sum or reference signals, the device will not produce power within the 5% wave shape deviation which is generally or typically the maximum acceptable deviation in the industry. Further, the gain of the overall machine may be severely limited by the electronic components of the mixer.
U.S. Pat. No. 3,070,740 (Chirgwin, et al) discloses a brush type machine in which excitation frequencies change and regulating circuitry causes the field of a generator to rotate continuously at a synchronous speed regardless of shaft speed. Thus, frequency variances were to be eliminated when shaft speed varied with the application or removal of substantial output loads to or from the output of the generator. Chirgwin et al teaches a frequency comparison scheme between a reference and the rotation rate in a pulse generator. Chirgwin et al, although ostensibly providing an output independent of shaft rotational rate, discloses a control method for a system having a designed isochronous prime mover and generator.
U.S. Pat. No. 3,183,431 (Ford) simiarly is an effort to provide a regulating system to control both the frequency and the voltage of the output of the generator by supplying variable frequencies to the poly phase rotor of the generating system to accommodate variances in shaft speed during operation.
Machines heretofore known, including those disclosed by Johnson, Ford and Chirgwin et al, use regulating schemes typically directed toward reducing or minimizing the lag time or response time to return the output frequency of the generating system to a design level in the presence of a sudden load change which causes prime mover output RPM to change. The schemes of Johnson, Ford and Chirgwin et al basically provide variable excitation by developing error signals having finite response times to regulate output frequency when used with a substantially isochronous prime mover output. Machines heretofore known, including the machines of Johnson, Ford and Chirgwin et al, do not provide for the optimization of prime mover operation while generating a stable output frequency.
Machines manufactured for the United States market have historically been designed to generate alternating current at a 60 hz frequency. Machines made in the United States for other parts of the world must be designed, redesigned or otherwise modified to generate output frequencies at the frequencies found throughout the world which include, for example, 55 and 25 hertz. Because of the different system frequencies used by various countries and industries throughout the world, machines designed for fixed 60 hz operation in the United States must be modified to operate with a different prime mover rotation rate to generate the frequencies required in the country or for the industry in which the machines are to be used. The result of such modifications is that the prime mover must operate at a speed other than designed speed resulting in a potentially significant loss of efficiency. In many cases, a machine may suffer significant power derating which in turn increases the capital cost per kilovolt-ampere or kilowatt hour.
There is a need for a generating system with a prime mover which may operate over a wide range of speeds during normal operation while at the same time generating an output at any number of preselected designed constant frequencies. Inherent savings in materials, repair time and the like may be obtained from such a machine or generating system.